You really need to cover more than the vents. The shell of the Phantom is open around the motors. Water could be easily blown down into the shell and short out the ESC's if they were not coated to be waterproof. The ends of the arms could be plugged with something like silicone putty to prevent water from getting in from the motor openings. ESC's can be cooled with heat sinks and do not really need outside air. I have built several quads that are waterproof and floating, using the Dex shells. I have had flight times up to 47 minutes without overheating the electronics.On a completely sealed, watertight shell, the barometer in the flight controller must be vented to atmosphere. You can use a Gortex or neoprene patch or a flexible diaphragm. If the compass is mounted separately on the landing gear, it will need to be waterproofed. A bit of dielectric grease will keep water out of the connector. If the compass is mounted in an external plastic housing with the GPS module, then it is a simple matter to seal up the plastic housing.
With the Phantom, I would be more worried about the camera and gimbal getting wet. These might tolerate a light rain as they are under the umbrella of the Phantom body. They would be difficult to seal watertight but not impossible.